Human corneal dystrophies and selected corneal degenerations which have been documented carefully in the patient are then, as specimens following corneal transplantations, studied in the laboratory in an effort to elucidate pathogenetic mechanisms. Morphologic examination by light and electron microscopy has provided further insight into cell-cell relationships in the normal and diseased states and about abnormal extracellular deposits, such as the newly described crystals in a patient with benign monoclonal gammopathy. Tissue and cell culture studies have revealed the in vitro proliferative patterns of corneal cells, including the epithelialization of the endothelial layer in corneas of patients with posterior polymorphous dystrophy. The presence and production of collagen and glycoconjugates and of collagenase has been probed with immunofluroescent, electrophoretic, and chromatographic procedures, with emphasis to date on abnormalities of collagen production and collagenase activity in keratoconus.